Field of the Invention
The present invention concerns a method for correcting respiratory influences on recordings of an examination object acquired by operation of of a magnetic resonance apparatus, and an electronically readable data storage medium encoded with programming instructions and a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance tomography (MRT) is an imaging modality that allows the high resolution generation of sectional images of living organisms such as humans. The patient is positioned in a homogeneous magnetic field B0. Gradient coils are used to modify the external magnetic field in the field of view (FOV) such that a body slice is selected and such that the generated magnetic resonance (MR) signals are spatially encoded. The acquired MR signals are entered as complex numbers into a memory that represents a mathematical domain called k-space. Each such complex number entered as a k-space data entry point has a magnitude and a phase. The subsequent reconstruction of the MR signals entered into k-space, for example using a Fourier transformation, produces an image of the selected slice that is used for the medical diagnosis. The MR signals are generated and detected using a radio-frequency system, which includes a transmit antenna that radiates radio-frequency (RF) excitation pulses into the patient, and a receive antenna, which detects the emitted RF resonance signals and forwards them for image reconstruction. The selection of a suitable pulse sequence, such as a spin echo sequence or a gradient echo sequence, and the associated sequence parameters, allow the contrast of the MR images to be varied in many ways depending on the diagnostic purpose. MRT maps body structures and is therefore a structural imaging method.
Movements during an MR recording(data acquisition), for example respiratory movements of a patient who is to be examined using MR, can result in artifacts, for example types known as ghosting, blurring and/or loss of intensity in the generated images, as well as registration errors between generated images in magnetic resonance imaging, particularly when examining the organs of the thorax and abdomen and other examination regions influenced by the respiratory movement of the patient. Such artifacts can make it difficult for a physician to make a diagnosis based on such images and can lead to lesions being overlooked.
Numerous techniques are known for reducing artifacts as a result of respiratory movement. One of these techniques is the emission of a trigger signal for acquiring magnetic resonance image data as a function of a respiratory movement, generally known as respiratory gating. Respiratory gating is a technique with which the breathing of the patient is detected during the MR measurement and assigned to the acquired measurement data. With respiratory gating the measurement data are only used for reconstruction if the detected respiratory movement associated therewith satisfies certain predeterminable criteria.
The patient's breathing can be detected using external sensors, e.g. a pneumatic cuff, or using MR signals, known as navigators. A navigator is generally a short sequence in which MR signals are acquired, for example from the diaphragm or another signal source in the examination object, the movement of which is correlated with the patient's breathing. The respiratory movement can be tracked by the position of the diaphragm or the other signal source represented by the navigator.
A phase navigator can be used to obtain a respiratory signal or a correction signal from a phase difference between two non-phase-encoded navigator signals recorded shortly after one another. Susceptibility changes produced by lung movement ultimately result in different phase differences between the respective navigators. Because the susceptibility changes still have an effect a significant distance away, a phase navigator acquisition can also take place in an imaging slice and does not have to be specially positioned.
With respiratory gating using navigators, the navigator sequence can be interleaved with the imaging sequence, and the diaphragm position measured with a navigator is then assigned to the imaging data acquired immediately after or before.
A distinction is made between retrospective and prospective respiratory gating.
With retrospective respiratory gating the respiratory movement is detected and recorded but not evaluated during the MR measurement. Rather, the data entry points in the memory representing k space are filled a number of times. Only some of the measured data is used for reconstruction, preferably the measured data for which the respiratory signal lies within a specified window around a marked respiratory position. When a specified k space data point required for image reconstruction has been measured (i.e., an MR data entry is made thereat) a number of times within the marked window, the data can be averaged. However when a data point has always been measured outside the window, the data point with the smallest deviation from the marked position can be used for reconstruction.
With prospective respiratory gating, the physiological respiratory signal measured with the use of a respiratory sensor (e.g. the diaphragm position measured using a navigator sequence) is evaluated during measurement and the MR measurement is controlled based on the detected physiological signal. In the simplest embodiment, known as the acceptance/rejection algorithm (ARA), the measurement of an imaging data packet (and in some instances the assigned navigator sequence) is repeated until the physiological signal falls within a previously defined acceptance window.
A further option for reducing artifacts is to implement the reconstruction algorithm with a movement compensation. Here the image data are segmented into states of different respiratory stages after a respiratory process has been detected. After the images for the corresponding respiratory stages have been reconstructed, a movement model is estimated by image registration and this is used in turn to reconstruct a movement-free image volume.
While signals measured using external sensors do not take into account the distance between MR slices and the lung or longer-term phase effects, signals acquired directly by an MR measurement are frequently subject to noise and phase errors and are often only reliable for a limited number of slice positions.